1. Field of the Invention
The invention relates to a hearing aid in which acoustic and/or mechanical feedback of the signal is compensated by an internal signal path. In particular, the invention relates to a hearing aid in which the signal path contains, in succession, a microphone, an A/D converter for conversion of the microphone output signal into a sequence of discrete digital samples, a signal processing stage, a D/A converter for converting the processed digital signals back into analog form, an amplifier and an output converter, and which is furthermore provided with a feedback path within the hearing aid, in which a digital filter with a finite impulse response is located, with a transfer function which can be set by setting corresponding filter coefficients, and a determination and setting circuit which determines the transfer function of the feedback signal path via which unwanted acoustic and/or mechanical feedback between the output converter and the microphone takes place, and which adjusts the filter coefficients of the filter in the feedback path within the hearing aid depending on the determined transfer function of the feedback signal path, such that this filter compensates, at least partially, for the acoustic and/or mechanical feedback.
2. Description of Related Art
A hearing aid of the type to which this invention is directed is described in European Patent Application Publication No. 0 415 677 A2. The disclosed hearing aid is of the type conventionally worn behind or in the ear, and with which the output signal reaches the wearer acoustically.
Most of the properties of the hearing aid described in patent application 0 415 677 can be applied by one skilled in the art to the case of a fully or partially implanted hearing aid, but there are also characteristic differences to which reference is made separately in this description. In particular, for implanted hearing aids, the user does not receive the output signal acoustically through the air, but it is generally coupled by an electromechanical converter to one of the auditory ossicles. Hereinafter, when the output converter of the hearing aid is addressed, it is always assumed that depending on the application it can be both an electroacoustical and also an electromechanical converter.
In the simplest case, as shown in FIG. 2, a hearing aid is comprised of a microphone 1 which receives an acoustic input signal ea(t) and converts it into an electrical signal e(t), a filter 4 which processes the signal e(t), such as is necessary for the special hearing damage of the wearer, and delivers an output signal a(t), an amplifier 6 which produces an amplified output signal av(t) therefrom, and an output converter 7. The letters (t) indicate that the signals are analog signals in the continuous time domain.
This principle is preserved if the signal path in the hearing aid is subjected to digital signal processing, as is shown in FIG. 3, in which case an analog/digital converter 2, which converts electrical output signal e(t) of microphone 1 into a sequence of discrete digital samples e(m), is added to the block diagram. The A/D converter 2 is followed by a digital filter 4 with a mode of operation which can be ignored here, in which samples e(m) are processed such as is necessary for the special hearing damage of the wearer. The letter (m) indicates that the signals are digital signals in a discrete time interval. This is followed by conversion of the filtered digital signals a(m) back into analog form using a digital/analog converter 5, after which, as before, follow amplifier 6 and converter 7. Otherwise it is essentially irrelevant whether D/A converter 5 and amplifier 6 are in fact separate units, or whether they are inseparably interconnected in a single unit.
Unfortunately, in practice, it usually cannot be avoided that the output signal aa(t) couples back to the microphone and that, therefore, a feedback signal r(t) is added to the acoustic input signal which is formed from signal aa(t) by the time behavior h(t) of feedback section 8. This yields the block diagram in FIG. 4.
In a conventional hearing aid, the feedback path leads through the air to the microphone, while in an implanted hearing aid there are different propagation paths, for example, via the bones and other parts of the skull, or on a path via the eardrum and air.
In such closed signal loops, it fundamentally applies that the signal becomes unstable as soon as the loop gain exceeds 1. But, before this limit is reached, at the frequencies at which the loop gain approaches 1, resonant phenomena occur which are unpleasant for the user of the hearing aid. Therefore, the loop gain should always remain essentially less than 1. However, this conflicts with the fact that, depending on the severity of the hearing damage of the wearer, under certain circumstances very high gains are necessary.
Not shown in the diagrams in FIGS. 2 and 3, however generally representing the prior art, there is digital system control which normally can be accessed via a remote control and which allows the properties of the hearing aid to be controlled, for example, the properties of filter 4 or amplifier 6. Moreover, in the operation of the hearing aid, the system control assumes control and monitoring functions in and between the individual modules.
It is prior art to at least partially compensate feedback according to FIG. 1 by internal feedback filter 9 in the hearing aid. This filter leads back from the input of D/A converter 5 to a summation point 3 at the output of A/D converter 2. So that undesired feedback is optimally compensated, filter 9 must, as accurately as possible, have the same signal behavior as the signal path 5, 6, 7, 8, 1 , 2, but with the opposite sign. Then, from digital signal a(m) on the path 5, 6, 7, 8, 1, 2 and on the path via 9, two oppositely identical digital signals form which cancel one another at the summation point 3. Thus, there remains only one digital signal which, in the ideal case, is exactly the digital representation e(m) of the acoustic input signal ea(t).
Thus, the problem exists of determining the transfer properties of filter 9 such that it has the same impulse response as the signal path 5, 6, 7, 8, 1, 2, but with the opposite sign.
This problem was solved, for example, according to European Patent Application No. 0 415 677 by a digital pseudo-noise signal being supplied at the output of digital filter 4. This noise signal travels both through the signal path 5, 6, 7, 8, 1, 2 and also through the filter 9. With optimum compensation, it would have to be exactly compensated at the summation point 3. To do this, the original digital noise signal is supplied to one input of a digital correlator while the output signal of the summation element 3 is supplied to the other input. The individual delay stages of the correlator deliver digital values which are used for adaptive optimization of the coefficient of the filter 9.
This process causes continuous matching of the filter to the conditions of feedback path 8 which are highly variable in time in conventional hearing aids. For example, shifting the hearing aid to behind the ear or approaching a sound-reflecting article can cause a significant change of the feedback path. The disadvantage of this process is a comparatively high cost in digital processing. Thus, for example, for one coefficient multiplication in the FIR digital filter at least two more multiplications with variable factors are required for filter adaptation.